1. Field of the Invention
The present invention is broadly concerned with a fluid system equipped with a shear-type relief device especially designed for shearing when system pressures exceed a predetermined level. More particularly, it is concerned with such a fluid system, such as found in shock-absorbing bumpers, where the shear-type relief device is capable of allowing fluid bleed therethrough under low pressure conditions but is operable under very high pressures to provide full pressure relief where prior devices have failed under these operating parameters.
2. Description of the Prior Art
Under existing governmental regulations, automobiles must be provided with shock absorbing bumpers which are capable of sustaining an impact of up to five miles per hour against a stationary object without bumper damage. One such bumper in widespread use includes, for each bumper, a pair of forwardly projecting fluid cylinders fixedly secured to the vehicle frame, as well as corresponding tubular pistons secured to the bumper and telescopically received by the cylinders. The pistons are equipped with a slidable seal intermediate the ends thereof, as well as an apertured, innermost cap. Pressurized gas (e.g., nitrogen) is located in a chamber between the bumper and piston seal, whereas a hydraulic fluid in liquid form is provided within the remainder of the piston and the cylinder, communication between these regions being established via the cap aperture. In addition, an elongated, tapered central rod is located within the cylinder and extends through the cap aperture into the tubular piston. An oppositely tapered grommet presenting an orifice is located within the cap aperture in circumscribing relationship to the stationary rod.
When a vehicle having a shock absorbing bumper of the above type strikes an object, the pistons telescope into the corresponding cylinders, with the effect that the pistons shift along the length of the corresponding rods and the pressure of the hydraulic fluid is increased. An additional quantity of fluid is thus caused to pass from the cylinder into the piston through the central cap aperture, which in turn shifts the internal piston seal and compresses the gaseous material within the forward chamber. Flow of fluid through the restricted orifice and compression of the gaseous material creates a shock-absorbing resistance and thus prevents damage to the bumper.
While bumpers of the type described are indeed capable of withstanding impact loads of low or moderate magnitude, under high impact loads they typically fail and contribute to the possibility of intrusion of components of the vehicle itself into the passenger compartment thereby increasing the potential of occupant injury. This occurs under high impact pressure induced loads on the piston because of inability of the fluid to flow through the orifice defined by the grommet at a rate to allow the piston and cylinder assembly to function as an energy absorption device. Under certain extreme conditions, it is believed that as the bumper-mounted pistons move into their associated cylinders along the length of the tapered, stationary rod, the circumscribing grommets may in fact engage the rod and thereby be deformed and extruded into the cap aperture in such manner that the aperture is completely closed. When grommet collapse or limitation of fluid flow therethrough occurs, the system is no longer capable of any shock absorption whatsoever, and the full impact load is thus transmitted through the bumper to the vehicle frame.
It will be appreciated, however, that any proposed solution to the above problem must not interfere with the normal operation of the shock absorbing bumpers, and further must be entirely predictable in use and be relatively inexpensive. Thus, a bumper system having high impact shock absorption would be of little value if in accomplishing this goal shock absorption at low and moderate impact loads is destroyed or seriously impaired.